1. Field of the Invention
The present invention relates to tracking techniques of tracking acquired synchronization in a CDMA (Code Division Multiple Access) system.
2. Description of the Related Art
In CDMA mobile communication systems, it is necessary to always synchronize the despreading code sequence at a receiving side with the spreading code sequence at a transmitting side so that a reception signal is precisely demodulated at a CDMA receiver. Since a reception signal after acquisition is not always synchronized with the despreading code timing, tracking control is needed after the code acquisition has been completed.
As a conventional tracking control method, a CDMA chip synchronization circuit has been disclosed in U.S. Pat. No. 6,188,679 B1. This CDMA chip synchronization circuit is provided with multiple correlators to output correlation values each obtained for phased despreading codes. When a reception signal after acquisition is over-sampled at a multiple of the chip rate fc to produce reception data, as shown in FIGS. 9A-9D five correlators are provided to cope with delays of the timing which correspond to ±½ chip and +¼ chip from the center position (±0). Then, a correlation value having a best receiving quality is selected from the five correlation values, which are provided with certain intervals of time. In the case as shown in FIGS. 9C and 9D, the selected correlation value is obtained by a correlator corresponding to +¼-chip phase
In the above conventional tracking technique, however it is necessary to obtain the correlation values or powers at all phases. This causes the amount of circuit hardware to become larger as the over-oversampling factor increases- In addition, such calculations of correlation values or powers are performed in units of high-speed chip duration. Accordingly, power consumption required for tracking control is considerably increased as the over-sampling factor increases.